Wednesday, October 8, 2008
Tuesday, October 7, 2008
Current and future developments
Semiconductor corporations have worked to reduce the cost of the components in a flash drive by integrating various flash drive functions in a single chip, thereby reducing the part-count and overall package-cost.
Flash drive capacities on the market increase continually. As of 2008[update] few manufacturers continue to produce models of 256 MB and smaller; and many have started to phase out 512 MB capacity flash memory. High-speed has become a standard for modern flash drives and capacities of up to 64 GB have come on the market.[24]
Lexar is attempting to introduce a USB FlashCard [25] [26], which would be a compact USB flash drive intended to replace various kinds of flash memory cards. Pretec introduced a similar card, which also plugs into every USB port, but is just one quarter the thickness of the Lexar model [27] SanDisk has a product called SD Plus, which is a SecureDigital card with a USB connector.[28]
SanDisk has also introduced a new technology to allow controlled storage and usage of copyrighted materials on flash drives, primarily for use by students. This technology is termed FlashCP.
Comparison with other portable storage
Obsolete devices
Audio tape cassettes are no longer used for data storage. High-capacity floppy discs (e.g. Imation SuperDisk), and other forms of drives with removable magnetic media such as the Iomega Zip and Jaz drives are now obsolete and no longer an option.
Tape
The applications of current data tape cartridges hardly overlap those of flash drives: the drives and media are very expensive, have very high capacity, slower transfer speed than most other storage media, and store data sequentially, leading to very long access times. These devices are used for routine backup of large systems.
Floppy disk
Floppy disks are rarely fitted to modern computers and are obsolete for normal purposes, although internal and external drives can be fitted if required. Floppy discs may be the method of choice for transferring data to and from very old computers without USB or network support. Computers can usually boot from floppy discs, which can be a convenient way of updating flashable BIOS chips, etc.
Optical media
The various writable and rewritable forms of CD and DVD are portable storage media supported by the vast majority of computers as of 2008. CD-R, DVD-R, and DVD+R can be written to only once., RW varieties up to about 1,000 erase/write cycles, while modern NAND-based flash drives often last for 500,000 or more erase/write cycles.[citation needed] DVD-RAM discs are the most suitable optical discs for data storage involving much rewriting.
Optical storage devices are among the cheapest methods of mass data storage after the hard drive. They are slower than their flash-based counterparts. Standard 12 cm optical discs are larger than flash drives and more subject to damage. Smaller optical media do exist, such as business card CD-Rs which have the same dimensions as a credit card, and the slightly less convenient but higher capacity 8 cm recordable CD/DVDs. The small discs are more expensive than the standard size, and do not work in all drives.
Universal Disk Format (UDF) version 1.50 and above has facilities to support rewritable discs like sparing tables and virtual allocation tables, spreading usage over the entire surface of a disc and maximising life, but many older operating systems do not support this format. Packet-writing utilities such as DirectCD and InCD are available but produce discs that are not universally readable (although based on the UDF standard). The Mount Rainier standard addresses this shortcoming in CD-RW media by running the older file systems on top of it and performing defect management for those standards, but it requires support from both the CD/DVD burner and the operating system. Many drives made today do not support Mount Rainier, and many older operating systems such as Windows XP and below, and Linux kernels older than 2.6.2, do not support it (later versions do). Essentially CDs/DVDs are a good way to record a great deal of information cheaply and have the advantage of being readable by most standalone players, but they are poor at making ongoing small changes to a large collection of information; flash drives' ability to do this is their major advantage.
Advantages and disadvantages
Advantages
Flash drives are impervious to scratches and dust, and mechanically very robust making them suitable for transporting data from place to place and keeping it readily at hand. Most personal computers support USB as of 2008[update].
Flash drives also store data relatively densely compared to many removable media. In mid-2008, 64 GB drives became available, with the ability to hold many times more data than a DVD.
Compared to hard drives, flash drives use little power, have no fragile moving parts, and for low capacities are small and light.
Flash drives implement the USB mass storage device class so that most modern operating systems can read and write to them without installing device drivers. The flash drives present a simple block-structured logical unit to the host operating system, hiding the individual complex implementation details of the various underlying flash memory devices. The operating system can use any file system or block addressing scheme. Some computers can boot up from flash drives.
Some flash drives retain their memory after being submerged in water [17], even through a machine wash, although this is not a design feature and not to be relied upon. Leaving the flash drive out to dry completely before allowing current to run through it has been known to result in a working drive with no future problems. Channel Five's Gadget Show cooked a flash drive with propane, froze it with dry ice, submerged it in various acidic liquids, ran over it with a jeep and fired it against a wall with a mortar. A company specializing in recovering lost data from computer drives managed to recover all the data on the drive. [18] All data on the other removal storage devices tested, using optical or magnetic technologies, were destroyed.
Disadvantages
Like all flash memory devices, flash drives can sustain only a limited number of write and erase cycles before failure[19][20]. This should be a consideration when using a flash drive to run application software or an operating system. To address this, as well as space limitations, some developers have produced special versions of operating systems (such as Linux in Live USB) [21] or commonplace applications (such as Mozilla Firefox) designed to run from flash drives. These are typically optimized for size and configured to place temporary or intermediate files in the computer's main RAM rather than store them temporarily on the flash drive.
Most USB flash drives do not include a write-protect mechanism, although some have a switch on the housing of the drive itself to keep the host computer from writing or modifying data on the drive. Write-protection makes a device suitable for repairing virus-contaminated host computers without risk of infecting the USB flash drive itself.
A drawback to the small size is that they are easily misplaced, left behind, or otherwise lost. This is a particular problem if the data they contain are sensitive (see data security). As a consequence, some manufacturers have added encryption hardware to their drives -- although software encryption systems achieve the same thing, and are universally available for all USB flash drives. Others just have the possibility of being attached to keychains, necklaces and lanyards.
Compared to other portable storage, particularly external hard drives, USB flash drives have a high price per unit of storage and are only available in comparatively small capacities; but in the smaller capacities (4 GB and less), USB flash drives are much less expensive per unit of storage than the hard drives they have replaced.[22]
Size and style of packaging
Some manufacturers differentiate their products by using elaborate housings, which are often bulky and make the drive difficult to connect to the USB port. Because the USB port connectors on a computer housing are often closely spaced, plugging a flash drive into a USB port may block an adjacent port. Such devices may only carry the USB logo if sold with a separate extension cable.
USB flash drives have been integrated into other commonly-carried items such as watches, pens, and even the Swiss Army Knife; others have been fitted with novelty cases such as toy cars or LEGO bricks. The small size, robustness and cheapness of USB flash drives make them an increasingly popular peripheral for case modding.
Heavy or bulky flash drive packaging can make for unreliable operation when plugged directly into a USB port; this can be relieved by a USB extension cable. Such cables are USB-compatible, but do not conform to the USB 1.0 standard. [9] [10]
Design and implementation
One end of the device is fitted with a single male type-A USB connector. Inside the plastic casing is a small printed circuit board. Mounted on this board is some simple power circuitry and a small number of surface-mounted integrated circuits (ICs). Typically, one of these ICs provides an interface to the USB port, another drives the onboard memory, and the other is the flash memory.
Drives typically use the USB mass storage device class to communicate with the host.
|
| |
| 1 | USB connector |
|---|---|
| 2 | USB mass storage controller device |
| 3 | Test points |
| 4 | Flash memory chip |
| 5 | Crystal oscillator |
| 6 | LED |
| 7 | Write-protect switch |
| 8 | Space for second flash memory chip |
Essential components
There are typically four parts to a flash drive:
- Male type-A USB connector — provides an interface to the host computer.
- USB mass storage controller — implements the USB host controller. The controller contains a small microcontroller with a small amount of on-chip ROM and RAM.
- NAND flash memory chip — stores data. NAND flash is typically also used in digital cameras.
- Crystal oscillator — produces the device's main 12 MHz clock signal and controls the device's data output through a phase-locked loop.
Additional components
The typical device may also include:
- Jumpers and test pins — for testing during the flash drive's manufacturing or loading code into the microprocessor.
- LEDs — indicate data transfers or data reads and writes.
- Write-protect switches — indicate whether the device should be in "write-protection" mode.
- Unpopulated space — provides space to include a second memory chip. Having this second space allows the manufacturer to develop only one printed circuit board that can be used for more than one storage size device, to meet the needs of the market.
- USB connector cover or cap — reduces the risk of damage and prevents the ingress of fluff or other contaminants, and improves overall device appearance. Some flash drives do not feature a cap, but instead have retractable USB connectors. Other flash drives have a "swivel" cap that is permanently connected to the drive itself and eliminates the chance of losing the cap.
- Transport aid — the cap or the main body often contains a hole suitable for connection to a key chain or lanyard.
First commercial product
Trek Technology and IBM began selling the first USB flash drives commercially in 2000. Singaporean company Trek Technology sold a model dubbed the "ThumbDrive," and IBM marketed the first such drives in North America, with its product the "DiskOnKey" (which was manufactured by M-Systems). IBM's USB flash drive became available December 15, 2000[5][6], and had a storage capacity of 8 MB, more than five times the capacity of the (at the time) commonly used floppy disks.
In 2000 Lexar introduced a Compact Flash (CF) card with a USB connection, and a companion card read/writer and USB cable that eliminated the need for a USB hub.
In 2004 Trek Technology brought several lawsuits against other USB flash drive manufacturers and distributors in an attempt to assert its patent rights to the USB flash drive. A court in Singapore ordered competitors to cease selling similar products[7] that would be covered by Trek's patent, but a court in the United Kingdom revoked [8] one of Trek's patents in that country.
Second generation
Modern flash drives have USB 2.0 connectivity. However, they do not currently use the full 480 Mbit/s (60MB/s) the USB 2.0 Hi-Speed specification supports due to technical limitations inherent in NAND flash. The fastest drives currently available use a dual channel controller, although they still fall considerably short of the transfer rate possible from a current generation hard disk, or the maximum high speed USB throughput.
Typical overall file transfer speeds vary considerably, and should be checked before purchase; speeds may be given in megabytes or megabits per second. Typical fast drives claim to read at up to 30 megabytes/s (MB/s) and write at about half that. Older "USB full speed" 12 megabit/s devices are limited to a maximum of about 1 MB/s.
